Fusarium solani f. sp. eumartii, historically considered solely a pathogen of potato (Solanum tuberosum), was associated with tomato plants (Lycopersicon esculentum) exhibiting foot rot symptoms in California. The pathogenicity of California isolates of F. solani f. sp. eumartii from potato plants with Eumartii wilt symptoms and tomato plants with foot rot symptoms was determined on potato, tomato, pepper (Capsicum anuum), and eggplant (S. melongena). Isolates from both potato and tomato caused dry rot symptoms on potato tubers and root or collar rot on all four host species in the greenhouse. In field trials, isolates from both tomato and potato were pathogenic on tomato, potato, and pepper, confirming that the host range of F. solani f. sp. eumartii is not limited to potato. The phylogeny of isolates from potato and tomato was determined based on sequences of two DNA fragments: rDNA internal transcribed spacer regions and partial sequences of elongation factor 1-α. All of the California isolates of F. solani f. sp eumartii from tomato and potato formed a single monophyletic clade distinct from other formae speciales and mating populations of F. solani. The results of this study demonstrate that Eumartii wilt and tomato foot rot in California both are caused by F. solani f. sp. eumartii.